Showing papers on "Triplet state published in 1976"
217 citations
TL;DR: The magnetic state of iron in haem proteins has long been recognized as a convenient indicator of chemical coordination as well as more subtle biochemical properties, but these have only rarely been invoked in recent years as an explanation for experimental haem protein magnetic data.
Abstract: The magnetic state of iron in haem proteins has long been recognized as a convenient indicator of chemical coordination as well as more subtle biochemical properties. In the trivalent case in particular, there are three magnetically distinct configurations of the five 3d electrons, yielding spin states of S = 1/2, 3/2 and 5/2. The first and third are extremely familiar, and often lie close enough in energy so that a thermal mixture of low-spin S = 1/2 and high-spin S = 5/2 states exists in an ensemble of molecules. Selection rules for common perturbations (ΔS = ∘, ± 1 for the spin-orbit interaction and ΔS = ∘ for the electronic Zeeman interaction) ensure that quantum mixtures, in which the wave function is a true combination of S = 1/2 and S = 5/2 components, are not observed. The mid-spin, S = 3/2, state, and allowed 5/2–3/2 and 1/2–3/2 quantum mixtures including it, are much less well known. These have only rarely been invoked in recent years as an explanation for experimental haem protein magnetic data.
167 citations
TL;DR: It is suggested that the photoreduction of P-760 occurs under the interaction of reduced cytochrome c with the reaction center state P+-890-P--760 which is induced by light, which appears to be a primary product of light reaction in the bacterial reaction centers.
157 citations
TL;DR: Observations are consistent with a role of carotenoids in preventing the formation of singlet O2 in vivo and can be explained by the proposal that single-triplet fusion causes rapid quenching of excited single states in the antenna bacteriochlorophyll.
152 citations
TL;DR: Room temperature redox potentiometry in combination with EPR analysis at 7 K, of cytochrome c553 and the reaction center bacteriochlorophyll dimer (BChl)2 absorbing at 883 nm yields identical results to those previously reported using optical analytical techniques at 77 K, and reveals that one heme equivalent of cy tochrome becomes oxidized for the generation of the state.
127 citations
TL;DR: Characteristics suggest that singlet oxygen and a compound in the triplet state (probably a carbonyl compound) are generated by a self-reaction of lipid peroxy radicals in NADPH-dependent microsomal lipid peroxidation.
121 citations
94 citations
TL;DR: In this paper, the spin selectivity of the reverse electron transfer process within the intermediate charge transfer complexes is discussed in terms of the spin-selectivity of Bph* and BphT in the presence of 8 M methyl iodide and 40 mMp-benzoquinone.
Abstract: — Nanosecond and picosecond. kinetic techniques have been used to study electron transfer from the first excited singlet state (Bph*) and the first excited triplet state (BphT) of bacteriopheophytin to p-benzoquinone. Quenching of the first excited singlet state by 40 mMp-benzoquinone results in a decrease in the lifetime of Bph* but does not lead directly to the formation of the π-cation radical (Bph†). In the presence of 8 M methyl iodide and 40 mMp-benzoquinone together, the singlet lifetime is reduced further; however, the quantum yield of BphT is enhanced due to the increased rate of intersystem crossing between Bph* and BphT. Electron transfer from BphT to p-benzoquinone leads to the formation and detection of Bph†. The results are discussed in terms of the spin-selectivity of the reverse electron transfer process within the intermediate charge transfer complexes.
92 citations
TL;DR: Comparison of the carotenoid triplet energy levels with that of O2('Δg) suggests that spirilloxanthin, spheroidene and possibly alsoζ‐carotene, would be expected to protect against photodynamic action caused by O2 ('Γg), but not cis or trans phytoene.
Abstract: — Absorptions of the triplet excited states of five carotenoids (15,15'-ds phytoene, all-trans phytoene, C-carotene, spheroidene and spirilloxanthin), extracted from the photosynthetic bacteria Rhodopseudomonas spheroides and Rhodospirillum rubrum, have been detected in solution using pulse radiolysis and laser flash photolysis. Triplet lifetimes, extinction coefficients, lowest energy levels and quantum efficiencies of formation have been determined. Comparison of the carotenoid triplet energy levels with that of O2('Δg) suggests that spirilloxanthin, spheroidene and possibly alsoζ-carotene, would be expected to protect against photodynamic action caused by O2 ('Δg), but not cis or trans phytoene. The S→T intersystem crossing efficiences of all five polyenes were found to be low, being a few per cent or less. In their protective role these triplet states can only therefore be effectively reached via energy transfer from another triplet, except in the case of O2 ('Δg). The low crossover efficiencies also mean that light absorbed by such carotenoids in their possible role as accessory pigments would not be wasted in crossing over to the triplet state.
91 citations
TL;DR: In this article, the reduction of the triplet state of methylene blue by both alkyl- and aryl-amines was studied, and it was concluded that reaction of amines with amines leads to the formation of a partial charge-transfer intermediate which may undergo complete electron transfer to yield radicals, or undergo spin inversion and return to the ground state.
Abstract: — Flash photolysis was used to study the reduction of the triplet state of methylene blue by both alkyl- and aryl-amines. The extent of the formation of the semireduced form of the dye yielded rate constants of interaction between the triplet state and the amine (k5). A correlation between log k5 and ionization potentials for alkylamines (slope = -1.75 eV-1) was interpreted as evidence for the formation of a partial charge-transfer intermediate. The rate constants (k5) calculated for aryl-amines approached the rate of diffusion in many cases. A Hammett plot for a series of N, N-dimethyl-anilines yielded a moderately large p value (– 3.28) consistent with the formation of a charge-transfer intermediate. It was concluded that reaction of amines with triplet methylene blue leads to the formation of a partial charge-transfer intermediate which may undergo complete electron transfer to yield radicals, or undergo spin inversion and return to the ground state.
86 citations
TL;DR: Triplet state lifetimes of air-equilibrated solutions of eosin-protein conjugates are in the order of 10 mus, demonstrating that the probe is protected from oxygen quenching by the protein.
Abstract: Slow rotational diffusion may be investigated by measuring the decay of dichroism of flash-induced transient absorption changes of suitable probes. The preparation of the covalent ""triplets" probe eosin isothiocyanate is described together with investigations of spectroscopic properties of eosin-protein conjugates. Triplet state lifetimes of air-equilibrated solutions of eosin-protein conjugates are in the order of 10 mus, demonstrating that the probe is protected from oxygen quenching by the protein. Experiments with the sucrase-isomaltase complex from small intestine show that its enzymatic activity is little affected by binding up to 2 mol of eosin/mol of protein.
TL;DR: In this article, the triplet level for unsubstituted thiacarbocyanines can be populated efficiently owing to triplet-triplet energy transfer, and the quantum yield of intersystem crossing is low (ΦST?? 0.1).
TL;DR: In this article, the spin-orbit contribution to the zero-field splitting (ZFS) in the CH2O 3 A 2(n→π*) state is evaluated using the full Breit-Pauli Hamiltonian.
Abstract: The spin–orbit contribution to the zero‐field splitting (ZFS) in the CH2O 3 A 2(n→π*) state is evaluated using the full Breit–Pauli Hamiltonian. All calculations are carried out at the planar ground state geometry using a double‐zeta plus polarization basis of contracted Gaussian‐lobe functions augmented with diffuse s and p functions. Configuration–interaction wavefunctions, constructed using the 3 A 2 canonical orbitals, are used to describe the 3 A 2 state and all states coupling to it via the spin–orbit Hamiltonian. The excitation energies and oscillator strengths obtained from these wavefunctions are in good agreement with other theoretical calculations and with experiment. Of the 12 states considered in the second‐order perturbation theorytreatment of the spin–orbit interaction, the 1 A 1 ground and the nearby 3 A 1(π→π*) states were the most important. Rydberg states were observed to have very small spin–orbit matrix elements and consequently to have little effect on the ZFS. The spin–orbit contributions to the ZFS parameters D and E were −0.224 and 0.009 cm−1, respectively, which when added to the spin–spin contribution obtained in an earlier paper [S. R. Langhoff, S. T. Elbert, E. R. Davidson, Int. J. Quantum Chem. 7, 999 (1973)] give total values of D=0.314 cm−1 and E=0.04 cm−1. These results are larger than the best experimental results of D=0.141 cm−1 and E=0.02 cm−1, determined by Birss e t a l. [F. W. Birss, R. Y. Dong, and D. A. Ramsay, Chem. Phys. Lett. 18, 11 (1973)] from a rotational analysis of the 0+←0 bands of the 3 A 2←1 A 1 transition. An extensive calculation was also undertaken to assess the degree of convergence in the second‐order perturbation theorytreatment. The contribution of the lowest 100 singlet and triplet states of A 1, B 1, and B 2 symmetry were considered where each state was described by a 100‐term CI wavefunction. This calculation gives a spin–orbit contribution to D of −0.221 cm−1 essentially identical to the previous result providing evidence that the second‐order treatment has indeed converged. The radiative lifetimes of the three sublevels of the triplet state were determined using the same representations for the manifold of electronic states. In the high temperature limit, the radiative lifetime was determined to be between 0.02 and 0.06 sec, somewhat longer than the estimated experimental value of 0.01 sec. The mutual perturbation of the 1 A 1 ground and 3 A 2 states and the perturbation of the 3 A 2 state by the 1 A 1(π→π*) state were determined to be most important in determining the lifetime. These results ensure that the emitted light is polarized primarily along the carbon–oxygen bond in agreement with experiment. A critical examination of the quantitative validity of the numerical results is presented to assess the reliability of the theoretically determined lifetimes.
TL;DR: Optically detected ESR spectra in zero magnetic field of the triplet state of three photosynthetic bacteria are presented and the zero field splitting parameters and the widths of the resonances show small but significant differences for the three bacteria.
TL;DR: Phototropic and light growth responses of the sporangiophore of Phycomyces have been elicited using tunable laser stimulation from 575 to 630 nm, indicating a direct transition from the ground state to the lowest triplet state of riboflavin.
Abstract: Phototropic and light growth responses of the sporangiophore of Phycomyces have been elicited using tunable laser stimulation from 575 to 630 nm. The growth response shows additional components of the action spectrum with a sharp peak at 595 nm, a sharp cut-off at 585 nm, and a tail extending beyond 630 nm. The integral over the electronic transition (f-value) is 1.5 X 10(-9) times that at 455 nm. These parameters indicate a direct transition from the ground state to the lowest triplet state of riboflavin.
TL;DR: In this article, the phase memory time T M in the triplet state of the pair of two translationally inequivalent naphthalene molecules in an external magnetic field as a function of temperature was measured.
TL;DR: In this paper, the zero-field EPR transitions and spin sublevel intersystem crossing rates for the lowest triplet states of tetracene, perdeuterotetricene, 1,2,benzanthracene and 1, 2,3,4,dibenzanthricene have been measured in n−alkane solvents at 2°K by optical detection of magnetic resonance techniques.
Abstract: The zero‐field EPR transitions and spin sublevel intersystem crossing rates for the lowest triplet states of tetracene, perdeuterotetracene, 1,2‐benzanthracene, and 1,2,3,4‐dibenzanthracene have been measured in n‐alkane solvents at 2°K by optical detection of magnetic resonance techniques. The rates of spin sublevel intersystem crossing are primarily nonradiative, and an analysis of these molecules, as well as the triplet state dynamics of several groups of polycyclic hydrocarbons, is presented with conclusions applicable to planar aromatic organic systems in general. Comparison of the triplet state intersystem crossing rates of the polycyclic hydrocarbons suggests that nonradiative transitions for the individual spin sublevels of the lowest triplet state may be qualitatively understood and predicted by a determination of the symmetry‐selected one‐center spin‐orbit coupling mechanism available to each triplet sublevel.
TL;DR: The possibility of a triplet participation in the primary processes of photosynthesis is discussed, and the dependence of the delayed luminescence of Photosystem I on the state of the reaction centers has been studied.
TL;DR: In the presence of hydrogen donors the xanthone triplet state is shown to react to give Xanthone ketyl radical the absorption spectrum of which has been measured in several solvents.
Abstract: A strong transient absorption with a peak near 600 nm has been observed following laser photolysis of xanthone in fluid solution: this absorption is due to the first excited triplet state of xanthone and an estimate of its extinction coefficient is given. Triplet decay in various solvents depends upon solvent dielectric and hydrogen atom donating ability; however, an unusually short lifetime of 92 ns is noted with benzene as solvent. The quenching constants of triplet xanthone by naphthalene, oxygen and 3-methylindole are 9.5, 5.6 and 8.4 × 109 dm3 mol–1 s–1 respectively. Oxygen quenching is greater than the expected limiting value of one ninth diffusion controlled.In the presence of hydrogen donors the xanthone triplet state is shown to react to give xanthone ketyl radical the absorption spectrum of which has been measured in several solvents. The decay of this radical in the presence and absence of oxygen has also been investigated.
TL;DR: In this article, the triplet state zero-field splittings and rates of intersystem crossing for individual triplet spin sublevels in chlorophyll a, chlorophyck b, and zinc-substituted chlorophlls are presented.
Abstract: The triplet state zero-field splittings and rates of intersystem crossing for the individual triplet spin sublevels in chlorophyll a, chlorophyll b, and zinc-substituted chlorophylls are presented. Whereas there are only small differences in zero-field splitting parameters among the chlorophylls, their kinetics show significant differences. The general trends in the chlorophyll intersystem crossing rate constants of the triplet spin sublevels may be explained qualitatively by a consideration of the one-center spin--orbit and spin--vibronic coupling activity of the chlorophyll ring system and the effect of side-chain substituents on these coupling mechanisms.
TL;DR: The interaction between quinone and the chlorophyll excited singlet state results in energy dissipation without measurable formation of radical species having lifetimes longer than 10 ps, in marked contrast to the quenching of the chloropyll lowest triplet state by quinones, during which easily detectable stable radical formation has been observed.
TL;DR: The build-up of triplet-triplet absorption was measured for phenazine by the picosecond spectroscopy method as discussed by the authors, and the measured rate constants for the T 1 state are 7 × 10 and 5 × 10 10 5 −1 in iso-octane and in methanol re- spectively.
TL;DR: In this paper, the formation of hydrated electrons and H atoms from neutral aqueous solutions has been studied by excitation in its first and second excited singlet state, and it was shown that there are at least two pathways for electron formation, one in competition to internal conversion to the fluorescing state and another occurring after population of the fluorescenceing state, but most likely not from the triplet state.
TL;DR: In this paper, the authors reported the first observation of ESR signals exhibiting the CIDEP phenomenon from free radicals associated with photosystem II of chloroplasts and whole algae and provided additional evidence that triplet states may be involved as intermediates in the mechanism of photochemical electron transfer.
Abstract: THE primary photochemical reaction of photosynthesis is widely accepted to be a one-electron transfer from a chlorophyll (or bacteriochlorophyll) species to an acceptor entity1. Until recently it had been assumed that the excited state precursor to the electron-transfer reaction was a singlet state in each of the photosynthetic photosystems. This view is based on the fact that estimated fluorescence lifetimes2 of ∼ 7 ps at the reaction centre are much too short compared with intersystem crossing times of the order of 5 ns for chlorophyll in vitro3. Robinson3 pointed out, however, that the increase in fluorescence yield observed when the photosystem II (PS II) trap is closed is far less than that expected if photochemistry, such as electron transfer, were the sole quenching process. He and others4,5 have proposed the involvement of a triplet state in the mechanism of the photochemical electron transfer, but this suggestion has received little attention. Blankenship et al.6 reported an electron spin resonance (ESR) signal arising from photosystem I (PS I) which initially exhibited microwave emission due to chemically induced dynamic electron polarisation (CIDEP). They maintained that this observation indicated that the excited state precursor to the formation of the radical species must have been in a triplet state to account for the CIDEP phenomenon. We report here the first observation of ESR signals exhibiting the CIDEP phenomenon from free radicals associated with PS II of chloroplasts and whole algae. As discussed below, this provides additional evidence that triplet states may be involved as intermediates in the mechanism of photochemical electron transfer in photosynthesis.
TL;DR: Sinanoglu et al. as mentioned in this paper have made optical detection of magnetic resonance (ODMR) measurements on the enzyme alpha-chymotrypsin, as well as on its complex with the dye, proflavin.
Abstract: We have made optical detection of magnetic resonance (ODMR) measurements on the enzyme alpha-chymotrypsin, as well as on its complex with the dye, proflavin. Evidence that triplet-singlet energy transfer occurs in the complex is provided by the observation of characteristic tryptophan ODMR signals while monitoring the delayed fluorescence of the dye. The luminescence decay kinetics of the complex indicates that nontrivial triplet-singlet transfer originates from several (at least three) tryptophan residues of the enzyme. ODMR sensitivity can be enhanced by coupling the sublevels of a weakly radiative triplet state to a fluorescent dye which satisfies Forster's (Forster, T. (1948), Ann. Phys. (Leipzig) 2, 55; (1965), in Modern Quantum Chemistry, Istanbul Lectures, Part III, Sinanoglu, O., Ed., New York, N.Y., Academic Press, p 93) conditions for energy transfer.
TL;DR: In this paper, a line structure of the spectra of 1,2-benzypyrene and chrysene in bromobutyl at 4.2 K under laser excitation directly in the region of the T1 ← S0 transition has been studied.
TL;DR: In this article, the photodegradation mechanisms in xanthene dye-laser solutions are investigated at 77 K by the direct detection of a decrease in the concentration of dye molecules in the lowest excited triplet state (T 1 state) and of free radicals produced after degradation using an ESR technique under UV (3511 + 3638 A) or 5145-A laser irradiations.
Abstract: The photodegradation mechanisms in xanthene dye-laser solutions are investigated at 77 K by the direct detection of a decrease in the concentration of dye molecules in the lowest excited triplet state (T 1 state) and of free radicals produced after degradation using an ESR technique under UV (3511 + 3638 A) or 5145-A laser irradiations. It is shown that the ESR spectral shape of the radicals induced by an UV laser beam is quite different from that of those induced by a 5145-A beam. For those radicals, the irradiation time and power dependences, the effects of dye and solvent molecular structures and the role of the T 1 state by the addition of cyclooctatetraene (COT) quenchers are examined in some detail. As a result the two following laser-induced photochemical reactions are proposed: in UV laser- or flashlamp-pumped dye lasers, the C-H bond rupture of a solvent molecule due to the energy transfer of a dye molecule in a higher excited triplet state, which is produced by a T-T absorption of a dye molecule in the T 1 state, results in a radical and a leuco compound of the dye. On the other hand, in the case of the 5145-A laser excitation, a partially reversible change of a dye molecule in the T 1 state produced by only a one-photon absorption results in another radical which depends on the chromophoric structure of the dye.
TL;DR: An improved version of fast response ESR spectrometer was used to study the spectrum of e$sup -$/sub aq/ in several chemical systems as discussed by the authors, including carbonate, phosphite, hypophosphite, phenol, and hydroquinone.
Abstract: An improved version of fast response ESR spectrometer was used to study the spectrum of e$sup -$/sub aq/ in several chemical systems. The strong e$sup - $/sub aq/ signal at g = 2.00033 was found in the methanol and sulfite systems. Ethylene, formate, and tert-butyl alcohol systems gave larger signals, and bromide and ferrocyanide systems gave smaller signals than the sulfite systems. In systems of carbonate, phosphite, hypophosphite, phenol, and hydroquinone the e$sup -$/sub aq/ ESR signal appeared in emission. The proposed explanation for the presence of the emission signal is that in the reaction between e$sup -$/sub aq/ and the counter radical in these systems, the product molecule preference is for the triplet state. A discussion of this theory is given. (DDA)
TL;DR: In this article, a double excitation method was proposed for the lowest excited singlet state of anthracene, 9-methylanthracene and 9-phenylthracenes in ethanol.
TL;DR: In this paper, triplet state transitions of the photosynthetic bacteria Rhodospirillum Rubrum, Rhodopseudomonas Spheroides and Chromatium Vinosum in chemically reduced preparations have been observed by zero-field optical detection of magnetic resonance at 2 K.